Refactor some form validation, TODOs

apps/kassa/templates/kassa/register.html

@@ -9,8 +9,8 @@
             <p>Enter card and phone number to register a new membership or add card to an existing user.
                 <!--
                 Add no-user membership [DONE].
-                Add membership for new user and existing user.
-                Renew membership for existing user.
+                Add membership for new user and existing user [DONE].
+                Renew membership for existing user [DONE].
                 Register card to existing user [DONE].
                 Replace card for existing user [DONE]
                 -->